Implement Recipe#filepath method:

Returns the filepath of the recipe when it is built from a file.

spec/producer/core/recipe_spec.rb

@@ -8,11 +8,16 @@ module Producer::Core
     subject(:recipe)  { Recipe.new(code) }
 
     describe '.from_file' do
+      let (:filepath)   { fixture_path_for 'recipes/empty.rb' }
+      subject(:recipe)  { Recipe.from_file(filepath) }
+
       it 'builds a recipe whose code is read from given file path' do
-        filepath = fixture_path_for 'recipes/empty.rb'
-        recipe = Recipe.from_file(filepath)
         expect(recipe.code).to eq File.read(filepath)
       end
+
+      it 'builds a recipe whose file path is set from given file path' do
+        expect(recipe.filepath).to eq filepath
+      end
     end
 
     describe '#initialize' do